Underground formation testing is performed during drilling and geotechnical investigation of underground formations. Testing of such underground formations is important as the results of such examinations may determine, for example, if a driller proceeds with drilling and/or extraction. Since drilling operations are expensive, excessive drilling impacts the overall economic viability of drilling projects. There is a need, therefore, to minimize the amount of drilling and to obtain accurate information from the underground formations.
Different types of information may be obtained from the underground formations. One of the primary forms of information is obtained using actual samples of fluid from underneath the ground surface. Such samples, when they are obtained, are analyzed to determine constituents of the underground formation.
Determination of the underground fluid constituents is important in the exploration for trapped hydrocarbon reserves. Determination of oil, gas or mixtures of oil and gas are of importance in many areas of the world, and correct determination of the presence of the constituents is valuable.
Obtaining fluid samples from a formation requires a great deal of precision. This precise sampling is referred to as focused sampling. Focused sampling techniques are described in detail in U.S. Pat. No. 8,210,260 to Milkovisch et al. and U.S. Patent Publication No. 2010/0071898 to Corre et al., the contents of which are herein incorporated by reference. In focused sampling, fluid is pumped from a formation through a peripheral zone and/or a central zone of a wellbore wall. The fluid is drawn and/or pumped into two or more flowlines of a downhole tester. Oftentimes, the pumping pressure is desired to be adjusted at the peripheral or guard zone relative to the pumping pressure of the fluid at the central or sample zone. However, adjusting of pumping pressure results in increased complexity, weight and cost of the downhole tester. The increased weight and complexity are due to the existence of a second pump because each flowline is required to be coupled to a pump.